1. Field of the Invention
The present invention relates to a process for producing a persulfate such as ammonium persulfate, sodium persulfate and potassium persulfate. More particularly, the present invention is concerned with an industrially advantageous process for efficiently producing a persulfate such as ammonium persulfate, sodium persulfate and potassium persulfate that are widely employed as a polymerization initiator for polyvinyl chloride and polyacrylonitrile and as a treatment agent for a printed wiring board.
2. Description of the Related Arts
Ammonium persulfate, sodium persulfate and potassium persulfate are industrially important compounds that are widely employed as a polymerization initiator for various polymers including in particular, polyvinyl chloride and polyacrylonitrile, and as a treatment agent for a printed wiring board. As a general process for producing ammonium persulfate, there is known an electrolytic production process by the use of, as a starting raw material, an aqueous solution containing sulfuric acid and ammonium sulfate, that is, an aqueous solution of ammonium hydrogensulfate. In the above-stated process, ammonium persulfate is produced by an electrolytic production process, and the resultant ammonium persulfate is concentrated, separated and dried by vacuum crystallization, centrifugal filtration or the like to obtain a finished product. At this time, the solution containing the crystal (usually referred to as"mother liquor ") is mixed with the liquid produced at a cathode and is used as a starting raw material for an anode.
For instance, Japanese Patent Application Laid-Open No.34700/1980 (Sho-55) discloses a process for producing an alkali salt or an ammonium salt of persulfuric acid by the anodic oxidation, in a diaphragm cell, of an aqueous solution containing sulfate ions including a proton, alkali metal ions or ammonium ions. This process, however is not economical because of a low current efficiency being at most 80% , approximately in the final electrolysis. In addition, Japanese Patent Application Laid-Open No.198275/1982 (Sho-57) discloses a process for producing ammonium persulfate by the use of ammonium hydrogensulfate and an electrolysis accelerator. This process, however as is the case with the foregoing, can not be said to be economical because of a low current efficiency being at most 80% approximately in the final electrolysis.
On the other hand, as a general process for producing sodium persulfate or potassium persulfate, there is known a production process by the reaction of ammonium persulfate and sodium hydroxide or potassium hydroxide in the aqueous solution. It is necessary in the above-mentioned process that in the first place ammonium persulfate as a starting raw material be produced by electrolysis method, and the resultant ammonium persulfate be concentrated and separated by vacuum crystallization, centrifugal filtration or the like and then taken out as a crystal. At the same time, the solution having contained the crystal, as is the case with the foregoing, is mixed with the liquid produced at a cathode and is used as a starting raw material for an anode.
The ammonium persulfate in the form of crystal thus obtained is re-dissolved in the next step, and is transferred to the step of reaction with sodium hydroxide or potassium hydroxide. In the aforesaid reaction step, a solution containing sodium persulfate or potassium persulfate is produced, then is concentrated and separated by vacuum crystallization, centrifugal filtration or the like and is subsequently taken out as a crystal. As stated hereinbefore, the process for producing sodium persulfate or potassium persulfate by the reaction of ammonium persulfate and sodium hydroxide or potassium hydroxide necessitates quite a long production steps and a number of steps and moreover, lowers the yield of the objective sodium persulfate or potassium persulfate based on the ammonium persulfate, thereby making itself far from economically advantageous.
Under such circumstances, several attempts have been made to produce sodium persulfate and potassium persulfate by the direct electrolysis without passing through ammonium persulfate. For instance, Japanese Patent Application Laid-Open No. 56395/1975 (Sho-50) discloses a process for producing sodium persulfate by the use of sodium hydrogensulfate as a starting raw material, which process, however is impractical because of an extremely low current efficiency in the electrolysis. Moreover, Japanese Patent Application Laid-Open No.133196/1975 (Sho-50) discloses a process for producing potassium persulfate by the use of potassium hydrogensulfate as a starting raw material, which process, however is impractical because of an extremely low current efficiency in the electrolysis in spite of the necessity for the use of a special electrolytic cell and expensive titanium-made cathodes.
In addition, Japanese Patent Publication No. 31190/1980 (Sho-55) discloses a process for producing sodium persulfate by means of electrolysis through the use of a neutral starting raw material for an anode in the presence of ammonium ions, which process, however can not be said to be economical because of a low current efficiency being about 70 to 80% in the electrolysis. Further, the aforesaid process suffers such disadvantages that the ammonium ions contained in the objective crystal increase the content of nitrogen components in the objective sodium persulfate and that the process necessitates a minute and attentive cleaning step in order to satisfy the ordinary requirement for the quality of sodium persulfate as the finished product, namely a purity of at least 99% by weight and the content of nitrogen components of at most 0.1% by weight.
In spite of a number of efforts and endeavors having heretofore been directed towards the improvement of the production process, it is the real situation that an economical process for producing sodium persulfate or potassium persulfate has not yet been developed.